Remember that talking about vitamins that dissolve in their excess fat is far worse than the disadvantages. If indeed you are a person who likes to maintain leopard gecko, then we feel we are brothers, because we have the same hobby. This one animal has its own charm for people in several countries.
Obviously, you have to give vitamins to leopard geckos because, when they are nocturnal, they do not receive enough UV light and hence do not synthesize vitamin D3 like other reptiles (iguana or pogona, for example). And obviously you also need to supply calcium, otherwise, you will suffer EMO (metabolic bone disease). Remember that calcium and vitamin D3 always go hand in hand, because if there is no calcium D3 is not absorbed in the bone.
Now, most diurnal reptiles receive vitamin D3 from UV light, so when iguanas or lizards are exposed to sunlight to heat they synthesize D3 without realizing it (that also happens to us). Therefore, to maintain this type of reptile, it is very important to have UV lighting. However, leopard geckos are active at night, so most of the time they will NOT be exposed to sunlight, and that happens the same in captivity, even if we put them in UV lighting, it will be of no use because they will spend the night sleeping in they are. So we have to give them the powder vitamin.
It is recommended to put the plug (from the bottle) and mix three parts of calcium (without any vitamins) and some vitamins (for example, Reptivite). So, they lick it when they want, and they get all the calcium and a small portion of essential vitamins. Other people sprinkle food twice a week with calcium and one with vitamins or sprinkle only with vitamins and leave the cover with only calcium for the gecko to take whenever you want.
All of this is acceptable, but if you do the method I mentioned earlier, you should not mix calcium that has vitamin D3 with a mixture of vitamins that has more D3, because as said, it will cause hypervitaminosis D3 type. The only D3 that does not cause excess problems is that obtained with UV light because when it is not needed it becomes inactive. Other people sprinkle food twice a week with calcium and one with vitamins or sprinkle only with vitamins and leave the cover with only calcium for the gecko to take whenever you want.
The only D3 that does not cause excess problems is that obtained with UV light because when it is not needed it becomes inactive. But if you do the method I mentioned earlier you should NEVER mix calcium that has vitamin D3 with a mixture of vitamins that has more D3, because as you said, it will cause hypervitaminosis type D3. The only D3 that does not cause excess problems is that obtained with UV light because when it is not needed it becomes inactive.